It's A Slug's Game

Sea slugs sport some of nature’s most unreal adaptations, including solar-powered skin and disposable penises. Welcome to their weird world…

- Dr Helen Scales

The term ‘sea slug’ is applied to an assorted and flamboyant bunch of molluscs, all close relatives of snails that evolved a shell-free adult life. They are found almost anywhere in the sea, from rock pools to the deep sea, in tropical and temperate waters, and even in the Arctic and Antarctica.

SOLAR POWERED SEA SLUG

It looks like a tiny, cartoon sheep, but this is in fact one of more than 4,700 known species of sea slug that creep and occasionally swim through the ocean. Known as the leaf sheep (Costasiella kuroshimae), this sea slug lives around coral reefs, where it grazes on clumps of algae. The green colouration comes directly from its food – a leaf sheep retains the algae’s chloroplasts (tiny cellular structures that harness the sun’s energy to make sugars), and incorporates them into its skin, where they continue to photosynthesise. This trick of stealing chloroplasts, called ‘kleptoplasty’, stops the leaf sheep from starving when there’s no food around. It’s one of a range of weird and unique survival techniques that sea slugs have evolved.

WHEN SLUGS HUG

Sea slugs are what’s known as ‘simultaneous hermaphrodites’. Each individual has female and male sex organs, but they can’t mate with themselves, and have to swap sperm with another slug. The sea slug penis, shown in this close-up of a pair of mating sea slugs (